1. Field of the Invention
The present invention relates to a titanium diboride(TiB2) sintered body and a method for manufacture thereof, and in particular to a titanium diboride sintered body containing a silicon nitride as a sintering additive.
2. Description of the Background Art
Titanium diboride has a high melting point of xcx9c3000xc2x0 C., an excellent hardness of xcx9c24 GPa and a high Young""s modulus compared to other materials, and has a good abrasion resistance as well as an oxidation resistance at a high temperature. Therefore, titanium diboride is generally used as an armor material, a primary wall tile of a nuclear fusion furnace, a cutting tool, a wear-resistance material, etc. In addition, other structural ceramics have a low electrical conductivity, whereas titanium diboride has a high conductivity of xcx9c105xcexa9xe2x88x921cmxe2x88x921, so that titanium diboride has attracted much attention for the wide application. Further, since titanium diboride has an excellent corrosion resistance with respect to a metallic aluminum and Na3AlF6 which is a melting electrolytic solution in a hall-heroult cell used when refining aluminum, titanium diboride may be substituted with a metallic material for a cathode material. In addition, its excellent conductivity enables this material to be formed to complicated shapes by an electrical-discharge-machining, and consequently increasing the applicable range of this material.
Despite of the above-described characteristics of the titanium diboride, the development and application of the same are rather limited mainly because of difficulty in obtaining a fully dense body. Being a covalent material, the self-diffusion coefficient of the titanium diboride is extremely low, so that the mass transport for the densification is quite restricted. In addition, a thin oxygen-rich layer existing on the surface of the titanium diboride powder is known to be very detrimental to the densification. Therefore, pure titanium diboride has been densified at extremely high temperatures (xcx9c2000xc2x0 C.) even with an applied pressure during the sintering. However, the high processing temperature are too expensive, and overgrowth of the particles during the process may deteriorate mechanical properties such as strength, etc.
Therefore, studies on the titanium diboride have focused on enhancing the sinterability. To improve the sinterability of the titanium diboride, transition metals such as Fe, Ni, and Co have been used as a sintering aid. The good wettability of these metals with the titanium diboride remarkably lowered the densification temperature. The addition of carbon was also found to be effective in improving the densification behavier of the titanium diboride by eliminating the oxide layer existing on the surfaces of the starting powder.
Another problem for wider application for the conventional titanium diboride is the relatively low strength and fracture toughness. Large grain size of the specimens, due to the high densification temperature or the oxide layer at the grain boundary, have very adverse effects on the mechanical properties. In addition, the formation of microcracks around the large grains during cooling because of thermal anisotropy of the titanium diboride further deteriorate the properties. Lots of attempts were made to improve the mechanical properties of the titanium diboride by adding a second phase as reinforcements. Non-oxides such as SiC, TiC, and B4C or oxides such as alumina(Al2O3), zirconia(ZrO2) were used as reinforcing agents.
However, the addition of metals to titanium diboride deteriorates the unique properties of the ceramics. Despite significant improvement in the mechanical properties, the addition of large quantity of second phases tends to alter the properties.
Accordingly, it is an object of the present invention to provide a fully-dense and strong titanium diboride sintered body of enhanced sinterability by adding silicon nitride (Si3N4) as a sintering.aid and method for manufacturing thereof.
The titanium diboride is hot pressed at 1800xc2x0 C. with the addition of silicon nitride as a sintering aid. The amount of silicon nitride has significant influence on the sinterability and the mechanical properties of the titanium diboride. When a small amount (2.5 wt %) of silicon nitride is added, the silicon nitride reacts with TiO2 existing on the surface of titanium diboride powder to form TiN, BN, and amorphous SiO2. The elimination of the TiO2 suppresses the grain growth effectively, leading to the improvement in the densification of the titanium diboride. The formation of SiO2 is also beneficial for the densification. The mechanical properties, especially the flexural strength, are enhanced remarkably through these improvements in the sinterability and the microstructure.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.